The present invention relates to a reflow soldering method and a reflow soldering furnace for heating a printed board on which solder paste is printed and electronic parts, such as surface-mounted devices (hereinafter referred to as xe2x80x9cSMDsxe2x80x9d), mounted on the printed board and soldering the electronic parts to the board.
Reflow soldering is known as a mounting technique for electrically connecting and mechanically fixing electronic parts, such as SMDs, to a printed board. In a reflow soldering process, various SMDs are mounted on a printed board, on which solder paste is printed in advance, in a manner such that their leads are in alignment with pads of a thin film circuit on the printed board. Thereafter, the printed board is introduced into a reflow soldering furnace (hereinafter sometimes referred to as xe2x80x9creflow furnacexe2x80x9d) and heated, whereupon the solder paste is melted so that the SMDs are soldered to the printed board.
The reflow soldering furnace for carrying out this reflow soldering process comprises a furnace body that is provided with a conveyor for conveying the printed board. In the reflow soldering furnace body, preheating zones and a main heating zone (or reflow zone), which are defined by furnace walls, arranged in the conveying direction of the conveyor. The printed board and the SMDs thereon, as to-be-heated objects, are heated by means of heating means that are provided in the zones, individually. The heating means may be conventional heating devices, such as a hot-gas applier for blowing a hot gas against each to-be-heated object and a radiant-heat applier using a far infrared heater and the like.
In the preheating zones of the reflow soldering furnace, each to-be-heated object is heated to a temperature of 120 to 170xc2x0 C. to ease thermal shocks on the SMDs. In the main heating zone that follows the preheating zones, the to-be-heated object is heated to a temperature of 210 to 230xc2x0 C., which is higher than the melting point (180xc2x0 C.) of solder by 30 to 50xc2x0 C., whereby the solder is melted. The to-be-heated object delivered from the main heating zone is subjected to natural or forced cooling so that the solder solidifies, whereupon the reflow soldering is completed.
With the advance of diversification of electronic parts such as SMDs, there is an increasing demand for printed boards that are mounted with a large number of electronic parts of various types each. Accordingly, a large number of electronic parts with different sizes (or different heat capacities) are expected to be reflow-soldered to each printed board efficiently and securely. On the other hand, there are printed boards of various sizes. In some cases, electronic parts may be mounted on large-sized printed boards with large heat capacities. In consideration of these circumstances, electronic parts are expected to be reflow-soldered to various printed boards with high efficiency and reliability.
In the conventional reflow soldering process, the entire to-be-heated object is heated in the furnace in which the temperature is raised to a level higher than the melting point of solder by means of a hot gas or a combination of a hot gas and an infrared heater. If the heater output is not high enough for large-sized electronic parts with large heat capacities, however, the temperatures of the parts and their surroundings cannot be raised with ease. In some cases, therefore, joints (solder joints) between the printed board and leads of the electronic parts may not be able to be heated to a predetermined temperature, resulting in defective soldering.
The aforementioned underheating can be compensated with an increase of the hot gas temperature or the heater output. If this is done, however, those portions of the printed board which carry no electronic parts thereon or small-sized electronic parts with small heat capacities will overheat. In such a case, the thin film circuit on the printed board may be disconnected or cracked, and the small-sized parts may possibly be damaged or lowered in properties.
Accordingly, the object of the present invention is to provide a reflow soldering method and a reflow soldering furnace, capable of carrying out appropriate reflow soldering without entailing under- or overheating despite the differences in heat capacity between electronic parts mounted on a printed board.
In order to achieve the above object, according to the invention, there is provided a reflow soldering method for heating a to-be-heated object to a target temperature in one or more heating zones, comprising blowing a hot gas of a temperature lower than the target temperature against the to-be-heated object by using hot-gas applying means in the heating zones and applying radiant heat of a temperature higher than the target temperature to the to-be-heated object, thereby heating the to-be-heated object to the target temperature.
According to this reflow soldering method, electronic parts with a small heat capacity are cooled by means of the hot gas of the temperature lower than the target temperature for the heating zones, while electronic parts with a large heat capacity are heated to the target temperature by means of the radiant heat. By doing this, a plurality of electronic parts with different heat capacities can be soldered to a printed board when the temperature differences between the electronic parts are reduced.
In order to achieve the above object, according to the present invention, there is provided a reflow soldering method for preheating a to-be-heated object to a temperature lower than the melting point of solder in one or more preheating zones and then heating the to-be-heated object to the melting point of the solder in a main heating zone, comprising blowing a hot gas of a temperature lower than the target temperature for the preheating zones against the to-be-heated object by using hot-gas applying means in the preheating zones and applying radiant heat of a temperature higher than the target temperature to the to-be-heated object, thereby heating the to-be-heated object to the target temperature.
According to this reflow soldering method, electronic parts with a small heat capacity are cooled by means of the hot gas of the temperature lower than the target temperature for the preheating zones at least in the preheating zones, while electronic parts with a large heat capacity are heated to the target temperature by means of the radiant heat. By doing this, a plurality of electronic parts with different heat capacities can be soldered to a printed board in the main heating zone when the temperature differences between the electronic parts are reduced.
In the reflow soldering methods according to the invention described above, the heating of the to-be-heated object by means of the radiant heat includes joint use of far infrared rays with a wavelength of 2.5 to 5,000 xcexcm and infrared rays including near infrared rays with a wavelength of 0.75 to 2.5 xcexcm. In some cases, the joint use of the infrared rays and the far infrared rays may be an effective measure for further reduction of the temperature differences between the parts on the printed board. In general, the printed board easily absorbs infrared rays with a wavelength of 2.5 xcexcm or more, while the electronic parts on the printed board easily absorb infrared rays with a wavelength of less than 2.5 xcexcm. Thus, the printed board and the electronic parts thereon have their respective infrared absorption spectra. In consequence, the temperature differences between the printed board and the electronic parts can be further reduced by jointly using an infrared heater and a far infrared heater and controlling the ratio between the respective outputs of these heaters.
Preferably, the radiation spectra of the infrared heater should exhibit a maximum value within a wavelength region of less than 2.5 xcexcm, and further preferably, within a region from 1 to 2.5 xcexcm. On the other hand, the radiation spectra of the far infrared heater should preferably exhibit a maximum value within a wavelength region of 2.5 xcexcm or more, and further preferably, within a region from 5 to 8 xcexcm.
According to the invention, the target temperature is a temperature at which thermal equilibrium is established for the preheating zones by the heating by means of the radiant heat and the cooling by means of the hot gas. It is necessary, therefore, only that the temperature of the hot gas in the preheating zones be set to be lower than the temperature (target temperature) at which the thermal equilibrium is established. Good results can be obtained, in particular, when the hot gas temperature is adjusted to a value lower than the target temperature by about 20 to 60xc2x0 C. The target temperature for the main heating zone is the temperature at which the solder melts. Accordingly, the temperature of the hot gas in the main heating zone is set to be lower than the melting point of the solder. The hot gas temperature described herein is a temperature reached immediately before the hot gas is in contact with the printed board.
According to the reflow soldering method of the invention in which the heating by means of the radiant heat and the cooling by means of the hot gas are combined together, dispersions in the temperature of the to-be-heated object can be reduced to a level lower than that of the conventional method if one or more of the heating zones are used. The heating method according to the invention may be used for some or all of the preheating zones or a part or the whole of the main heating zone. Alternatively, the heating method of the invention may be used for both the preheating and main heating zones. The object of the present invention can be best achieved by using the heating method of the invention at least for the preheating zones.
Curves L1 and L2 of FIG. 5 show temperature changes in parts with large and small heat capacities observed when the to-be-heated object is heated by stages in the preheating zones and the main heating zone in a general reflow soldering process. In order to solder the parts on the printed board uniformly, a temperature difference T1 shown in FIG. 5, that is, the difference between the respective peak temperatures of the parts with large and small heat capacities, must be reduced. To attain this, it is advisable previously to reduce a temperature difference T2 for the preheating zones, that is, the temperature difference obtained before the peak temperatures are reached, by using the heating method of the invention for the preheating zones.
In order to achieve the above object, a reflow soldering furnace according to the invention comprises a reflow soldering furnace body including one or more heating zones defined by furnace walls, a hot-gas applier for blowing a hot gas of a temperature lower than a target temperature for the heating zones against a to-be-heated object in the heating zones, and a radiant-heat applier for applying radiant heat of a temperature higher than the target temperature to the to-be-heated object. This reflow soldering furnace has been developed in consideration of the fact that the speed of temperature rise depends on the heat capacity of the to-be-heated object. More specifically, this arrangement is based on the fact that the temperature of an electronic part with a large heat capacity cannot increase with ease, while the temperature of a small-sized electronic part or a printed board with a small heat capacity can increase easily. Another factor is that the larger the heat capacity, the less easily the temperature lowers in a cooling process, and vice versa.
In the reflow soldering furnace of the invention, if the hot gas of the temperature lower than the target temperature is applied to the to-be-heated object, then the object will be cooled. If the radiant heat of the temperature higher than the target temperature is applied to the to-be-heated object, in contrast with this, then the object will be heated to a suitable temperature for soldering. Thus, the temperature of electronic parts with a large heat capacity can be increased to the suitable level for soldering by means of the radiant heat, while small-capacity parts can be cooled and prevented from overheating by means of the low-temperature hot gas. By controlling the balance between the cooling and heating, various electronic parts with difference heat capacities can be heated uniformly to soldering temperature.
In order to achieve the above object, a reflow soldering furnace according to the invention comprises a reflow soldering furnace body including one or more preheating zones and a main heating zone defined by furnace walls, a hot-gas applier for blowing a hot gas of a temperature lower than a target temperature for the preheating zones against a to-be-heated object at least in the preheating zones, and a radiant-heat applier for applying radiant heat of a temperature higher than the target temperature to the to-be-heated object in the preheating zones.
According to the invention, moreover, the radiant-heat applier includes heaters of two types, an infrared heater for generating infrared rays and a far infrared heater for generating far infrared rays. The joint use of these heaters of two types is an effective measure for the reduction of the temperature differences between the individual parts on the printed board.
The following first, second, and third layouts are provided for the arrangement of the radiant-heat applier and the hot-gas applier for carrying out the heating method according to the present invention. These layouts can lessen the temperature dispersions of the individual parts of the printed board, as compared with the prior art. The first and second layouts, in particular, can produce desirable results for the achievement of the object of the invention.
According to the first layout, the radiant-heat applier and the hot-gas applier are arranged so that the application of the radiant heat and the blowing of the hot gas from above the printed board can be carried out simultaneously. The second layout is an example of an individual arrangement. According to this layout, the radiant-heat applier and the hot-gas applier are arranged separately from each other so that the hot gas is blown against the printed board after the radiant heat is applied from above the printed board. The third layout is another example of the individual arrangement. According to this layout, the hot-gas applier and the radiant-heat applier are arranged separately from each other so that the radiant heat is applied after the hot gas is blown from above the printed board.
In a reflow soldering furnace according to an aspect of the invention, an infrared heater for radiating infrared rays is used in the radiant-heat applier. It is advisable, in particular, to use an infrared heater that radiates near infrared rays with a peak wavelength of 1 to 2 xcexcm, in particular. In this case, a heater (hereinafter referred to as xe2x80x9chalogen lamp heaterxe2x80x9d) using a halogen lamp can be used as the infrared heater. Since the packages of the electronic parts mounted on the printed board easily absorb short wavelengths of 1 to 2 xcexcm, in particular, the aforesaid heater can be suitably used to increase the speed of temperature rise of large-sized electronic parts whose temperature cannot be easily increased. However, the present invention is not limited to the arrangement in which the halogen lamp heater is used as the radiant-heat applier. For example, a far infrared heater or a combination of a far infrared heater and a halogen heater (infrared heater) may be used as the radiant-heat applier. In the latter case, the temperature differences between the individual parts can be reduced by controlling the respective outputs of the two heaters according to the types of the electronic parts and the printed board.
In the case where the hot-gas applier and the radiant-heat applier are used in combination in a reflow soldering furnace that carries but reflow soldering in an inert gas, the temperature of the hot gas that flows through a hot-gas circulation path may possibly become too high under the influence of the radiant heat in the furnace or the like. Preferably, the reflow soldering furnace of this type should further comprises hot-gas circulating means for guiding the circulating hot gas to the outside of the furnace and a heat exchanger for cooling the hot gas, guided by the hot-gas circulating means, by means of a heat transfer medium such as the open air. This heat exchanger serves to restrain the temperature of the hot gas in the reflow soldering furnace from being raised by the radiant heat or the like.
According to the invention, the hot-gas circulating means includes a by-pass duct diverging from the reflow soldering furnace body and provided with a radiating wall for use as the heat exchanger. The hot gas that passes through the by-pass duct is cooled as it is subjected to heat exchange with the heat transfer medium, such as the open air, outside the furnace through the radiating wall of the duct. Thus, the heat exchanger restrains the temperature of the hot gas in the reflow soldering furnace from being raised by the radiant heat or the like.
According to the invention, moreover, the heat exchanger may be provided with heat dissipation accelerating means, such as an air-cooling fan, water jacket, or radiating fin. This accelerating means serves to accelerate the heat exchange between the gas (hot gas) in the furnace and the heat transfer medium outside the furnace, thereby restraining the rise of the hot gas temperature more effectively.
According to the present invention described above, under- or overheating of the to-be-heated objects can be restrained to ensure appropriate reflow soldering despite the differences in heat capacity between the printed board and the electronic parts mounted thereon.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.